<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .out {
        height: 50px;
        display: flex;
        border: 1px solid black;
    }
    
    .in {
        flex: 0 1 300px;
        width: 500px;
        background-color: #fff;
        border: 1px solid brown;
    }
</style>

<body>
    <div class="out">
        <div class="in"></div>
        <div class="in"></div>
        <div class="in"></div>
    </div>
</body>

</html>